Carpenters use geometry to measure the length, calculate area and find the circumference of objects. Carpenters rely on different math functions, including geometry, to put things together in the way that makes the most sense.
Without math, carpenters would simply be assembling things that could turn out in any shape. A carpenter may want to create a house, but a house must be composed of angles and measurements. Without the measurements, a house will turn out lopsided and will not be able to stand up straight. Things that are created without the use of geometry are often not structurally sound. It is often more difficult for carpenters to create things when there is no mathematical basis for the objects.
Carpenters must be skilled at math for their carpentry skills to flourish. Many carpenters who are experienced in the field will be able to look at something and get a nearly exact calculation for the mathematics of the structure. Another skill that is important for carpenters to have is the conversion of measurements. Not everything that they work on will be measured in the exact same way, and they must be familiar with how to change between the standard and metric systems.